1. Field of the Invention
The invention relates generally to multimedia communication systems which utilize endpoint devices, and more specifically to techniques for matching the video transmission rates of a plurality of endpoint devices.
2. Description of Prior Art
Video transmission rate matching is a technique which has been used to convert the bit rate of a first video bit stream to match that of a second video bit stream. This conversion may require bit rate reduction and/or bit rate increases. Among these two conversion processes, bit rate reduction is more critical, due to the fact that this process involves the removal of bits. Since these bits represent video information, the quality of a video signal having a reduced bit rate may be degraded. The amount of degradation is related to the manner in which the bit rate reduction is achieved.
With respect to bit rate increases, no information is removed from the video bit stream. Instead, such a rate increase requires the incorporation of additional bits into the bit stream. Although it would be desirable to add bits to the bit stream for the purpose of enhancing the video signal, such enhancements are often in the pel domain. For example, if an incoming video signal has been subjected to a process of data compression, the information discarded in the compression process is not stored. The information is lost, and cannot be recovered and added to the bit stream during the process of increasing the bit rate. Therefore, bit rate increases are implemented by adding "extra" bits to the bit stream, wherein these "extra" bits are not the same bits that may have been removed during a previous data compression step. Bits are added using conventional bit-stuffing patterns to occupy the additional bandwidth of a rate-increased video signal.
Video transmission rate matching has been applied to a wide variety of applications in the field of multimedia communication, especially in environments involving multipoint communication topologies. For example, in multimedia conferencing applications, a "bridge" or "multipoint control unit" (MCU) is typically employed. This MCU is a computer-controlled device which includes a multiplicity of communication ports. The ports may be selectively interconnected in a variety of ways to provide communication among a group of endpoint devices. MCUs are often used to establish multi-party conferences among a plurality of endpoint devices, and to establish multi-party conference calls. A state-of-the-art MCU is described in ITU Document H.243, "Procedures for Establishing Communication Between Three or More Audiovisual Terminals Using Digital Channels up to 2 Mbps". March 1993, and in ITU Document H.231, "Multipoint Control Units for Audiovisual systems Using Digital Channels up to 2 Mbps", March 1993.
Existing MCUs require all endpoint devices participating in a given multimedia conference to use the same video transmission rate. Typically, during the initial setting up of a multimedia conference, or at the time that it is desired to add an additional endpoint device to an existing conference, the MCU polls all of the endpoint devices to ascertain the video transmission rate or rates each endpoint device is equipped to implement. When the MCU polls a given endpoint device, the endpoint device transmits a capability code back to the MCU. The capability code includes information specifying the video bit rate transmission capabilities, audio bit rate transmission capabilities, etc., of a given endpoint device. These capability codes may also specify the compression algorithm or algorithms used by a given endpoint device.
Based upon the capability codes received by the MCU from the various endpoint devices, the MCU selects a minimum video bit rate to be used as the video transfer rate for the multimedia conference. The MCU sends a video rate signal to all endpoint devices which specifies use of this video transfer rate. Upon receipt of the video rate signal, the endpoint devices all operate using the same video transfer rate.
Use of the same video transfer rate for all endpoint devices participating in a given conference presents significant shortcomings. For example, in many cases, a conference participant may desire to receive high-quality video by using a relatively high video transfer rate, whereas other participants may want to use less expensive equipment offering a lower video transfer rate. State-of-the-art MCUs cannot implement a multimedia conference using a plurality of different video transfer rates. Therefore, it would be desirable to equip existing MCUs with a video transmission rate matching device. Through the use of video transmission rate matching techniques, video communication among a plurality of endpoint devices will be provided, even when the endpoint devices use different video transmission rates.
One video transmission rate matching method well-known to those skilled in the art is termed the transcoding method. Pursuant to the transcoding method, a compressed video bit stream having a first bit rate is fully decoded into a video space known as the pel domain. This fully-decoded bit stream, which may be conceptualized as a completely reconstructed video sequence, is then encoded into a video bit stream having a second bit rate.
The existing transcoding method is disadvantageous. Since decoding as well as encoding processes are required, transcoding is very time-consuming. As a practical matter, the time delay is at least twice that of the end-to-end encoding delay. Such a delay is not tolerable for applications requiring real-time communication, such as multimedia conferencing. A faster method of performing video bit rate matching is needed.